Insufficient development of formation exposing technology in the Eastern Siberia may be reasoned by the problem of identifying primary and secondary reasons why formation permeability value decreases. Besides, region seems perspective considering presence of oil transport system, developing deposits with significant recoverable reserves and requires more accurate expert control. In attempt to specify the reasons of permeability decrease while formation exposing, it is necessary to observe geological-and-physical characteristics of terrigenous formations of Nepa-Botuoba oil-and-gas bearing region, especially B10 formation which is the most prevalent over estimated region, amongst most common factors. B10 formation is characterized by complex structure and development of lithological, stratigraphic and tectonic factors in various degrees. The post-sedimentation strata transformation processes associated with secondary cementation caused major effect on reservoir spreading. Sandstone salinization was one of them. In addition, several responsible for reservoir characteristics lithotypes are distinguished in the formation. Reservoir characteristics of B10 formation are variable, depending on geological profile and deposit location. High value of formation water mineralization and halite presence could be referred to the main reasons of permeability decrease. Low-mineralized drilling fluids based on sodium chloride solution and their filtrate could quickly penetrate productive layer in high volume, bypassing the largest pore throats, usual for the first lithotype. Negative effect of such fluids on the high-permeable layers is low. However, it could decrease permeability of adjoined productive layers. Such layers are composed with aleurolite and silt sandstone, including sulphate and carbonate formations. Considering interrelation between formation exposing result, various lithological and petrophysical properties and formation fluids saturation, it is necessary to prevent infiltration of drilling mud and its filtrate to reservoir formation at the significant depth while maintaining water-based mud.
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